Lost compression - after head change

[Steve Jones]

It seems that all you have done is change the cylinder head from one type to another. If the engine had compression and ran before the change it seems to follow that the problem relates to the change of cylinder head. Before looking at some of the more involved/unlikely suggestions (assuming the engine did have compression and did run before the change), you need to look at exactly what you did when you changed the head and, particularly, the head gasket used. As Tony says above, a LC gasket will not work well with a HC head.

Steve

[Bob Olive]

Thanks again everyone, in view of Tony’s and Steve’s suggestion that the gasket may be wrong I’ll take it all off and check. I have had the head and a gasket set on the shelf for a few years now, (marked HC head/gasket set). I can’t remember where I bought it but probably at Beaulieu Autojumble or A7 Rally...
I hope it is as simple as wrong gasket, although a bit embarrassing if it is, as not long ago I had a similar case of wrong gasket on a ‘27 side-valve Riley which delayed getting it back on the road for some months...
(See my profile - not a good mechanic. Maybe I should consider giving it all up due to Annodominitis?).

[Ruairidh Dunford]

A low compression gasket will work in conjunction with a high compression head, in so far as it will seal and run. The problem comes because of the wider sections that stick out into the combustion area - they burn off.

L/C gaskets are much thicker than a H/C gasket and are a little more forgiving on a warped head - I carry one in all the cars as "get you home" spare - I have had to use the combination (h/c head + l/c gasket) once, hence why I know that it will work, at least for the 300 miles needed to get me home on that occasion!

Do let us know what the problem turns out to be - something simple I hope.

[Davemayle]

Hi Bob, Well I have just fitted an HC head this morning, using the same NGK plugs as yourself. I decided to use the starter for first run and to my surprise it span over but didn't fire. Battery not at its best and started to slow up so stopped. It would have certainly started with the LC head. I put a charger on and cranked it by hand (I normally hand start) and a couple of swings it was away and running fine so maybe head/plug combination it does need a decent spark. 
Where are you located Bob?
Thanks
Dave

["Slack Alice" Simon]

One thought, very unlikely:

a skimmed head combined with a thin gasket (and maybe badly fitted studs) meaning some head studs effectively too long, and so the the nuts are tight, but not really pulling the head tight on to the block. .

[Bob Olive]

OK - so here goes. First, many thanks to you all for all your tints and hips (as I think Jack French used to say) you certainly inspired (challenged) me to get up and get on with solving the problem. Taught me a few things which at my advanced years I ought to have been more aware of - FIRSTLY: don't ASSUME (it can make an ASS out of U and ME) so, just because I changed the head - which by the way, was done 'by the book' i.e head checked for weft and warp, good clean studs, new and correct HC gasket, checking all plugs leads in correct order, etc etc etc. that meant not starting was as a result of changing the head. Having said that, I always start the car by hand when doing 'out of season' static garage warm-ups, as much as on a point of principle as anything, and think I have a feel for 'normal' compression, hence my stating that compression was 'down' post head change. As a matter of fact I still think that, but can't prove it as I did not do any compression test prior to this incident...
SECONDLY: even though my carb has suffered only one blockage in some five or so years of fairly regular motoring (resulting from an over-enthusiastic filling at a filling station - I usually fill from cans at home - with fresh petrol of course) don't ASSUME that fuel to the banjo means fuel to the engine, which as you will see was at least part of the trouble here. I should have taken time to check the carb and more time to check the plugs for wetness - didn't - stupid boy Pike!

THIRDLY: listen to the advice of others, or better still use the forum (this is my first foray... driven by desperation - which could have been avoided had I not ASSUMED things). Thanks John, I did pour a little fresh fuel into each cylinder, and indeed a cough and a splutter - what a wonderfully apt term that is - was the result. It's all a bit noisier than before but I expected that.

I will be doing a compression test in the next few days when I have time, and ask for ideas on what sort of figure to expect? A local friend who has been into Sevens since 195*? reckons that around 100 psi could be likely obviously dependent upon wear etc.,
I did a quick test on No 1 (before coming in to sit by the woodturner and to be swathed in blankets and to slurp hot soup...) and registered 75psi.
Since this is my first forum post - and just to keep the thread going a little longer - does anyone have any views?

There we are, sorry to have caused you all unnecessary concern/thought/effort due to my silly ASSUMPTIONS, but it is much appreciated - and kept you from worrying about Breaksit, didn't it?

[John Mason]

Bob, I cannot give you figures for a compression test but what is important is that all four cylinders should be about the same give or take a few pounds. In my previous post I asked if the plugs were wet after cranking. This is a simple way to establish if petrol is getting to the combustion chamber of the engine where the plugs are.

Pleased to hear you have now got it sorted.

John Mason

[Roland Alcock]

Here’s an example of pressure results from my three bearing engine. 

September.     November 2016
1 115.               105
2 105.               105
3 90 - 120w.    105
4 110.               105

The difference of readings from September to November  is purely down to just using the car. 

Roly

[Bob Olive]

Thanks John, and Roland those figures bear out what my friend reckoned. On another note, I hope you escape the floods which are heading your way. I was at school at Hanley Castle, and lived in poor old water-logged Evesham (fortunately at the top of Cheltenham Road. Stay dry.

[Tony Press]

I wrote in September 2018 and I quote-


The standard Compression Ratios seem to be

1923-1933 4.8 (or 4.9) to 1

1933-1936 5.2 ? to 1

1936- 1939 6.0 (or 6.2?) to 1

I understand that Compression Pressures are between 17 and 20 times the Compression Ratio in a good engine giving :

4.8 to 1 = 80 to 96 psi

5.2 to 1 = 88 to 104 psi

6.0 to 1 = 102 to 120 psi   

I get around 100 psi from the standard early head and 110 psi from the 1936 head with dry hand cranking and throttle open.

As suggested all cylinders should be within around 3-5 psi of each other .